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. Author manuscript; available in PMC: 2020 Jul 1.
Published in final edited form as: Acc Chem Res. 2019 Jul 18;52(10):2870–2880. doi: 10.1021/acs.accounts.9b00189

Figure 6.

Figure 6.

The T4 lysozyme (T4L) model system for XB studies. a. The hydroxyl of the tyrosine amino acid at position 18 (Y18) forms an HB to the polypeptide backbone of glutamate E11 (dashes). The side chain of tyrosine at Y88, however, is solvent exposed and does not interact with the remainder of the protein. b. Replacing Y18 with a metachlorotyrosine (mClY18) maintains the essential HB to E11, with the addition of an XB from the Cl to the peptide oxygen of glycine G28. c. Electrostatic potentials of a chlorophenol model of the mClY18 side chain. The Cl substituent shows a weak σ-hole when the hydrogen of the OH is rotated away from the halogen (top) but becomes significantly enhanced when rotated to form an HB.